1. Field of the Invention
This invention relates to recording tape cassettes and particularly to cassettes for use with a scratch strain recorder of the type which has a drive shaft for advancing the tape and a stylus for scratching the tape when a body which is being monitored experiences strain.
2. Description of the Prior Art
U.S. Pat. No. 3,906,511 (Kelemen) which issued on Sept. 16, 1975 describes a scratch strain recorder of the type for which the cassette of the present invention is particularly suitable and the disclosure of that patent is hereby incorporated by reference.
Broadly the scratch strain recorder of the prior patent is arranged to be mounted to a body the strain of which is to be monitored. The recorder includes a rotatable drive shaft which rotates only when strain motion occurs and a stylus or needle which also moves when strain motion occurs. Although movement of the stylus can be in two opposite directions depending on the direction of the strain the drive shaft always rotates in the same direction regardless of the strain direction.
A cassette carrying a steel recording tape is mounted onto the recorder so that a capstan in the cassette is drivingly engaged by the recorder drive shaft. The cassette also has a pinch roller or back-up roller, the recording tape passing between the capstan and pinch roller on its way between a supply pin and a take-up pin. Both of these pins are non-rotatably mounted in the cassette and an opposite end of the tape is secured to each pin. The tape is wound as a spring coil, there being an equal number of turns of the coil wound around each pin. However, in an unused cassette the turns around the supply pin are very loosely wound while the turns around the take-up pin are tightly wound so that when the capstan rotates, on sensing by the recorder of strain motion, a small length of tape from the loosely wound coil around the supply pin is fed to the tightly wound coil around the take-up pin, thus making the supply coil tighter and the take-up coil looser. The recordable length of tape is effectively used up when the supply coil is pulled tightly around the supply pin.
The cassette disclosed in U.S. Pat. No. 3,906,511 operates satisfactorily in that it provides very low resistance to feeding of the tape by the capstan and thereby permits smooth and accurate feeding of the tape by the capstan even when the capstan is subjected to rapid intermittent rotation. A major problem with the prior cassette, however, is that because of the amount of tape required to be wound around the pins to enable take-up and supply of tape to function, the amount of tape available for recording is greatly reduced e.g. in a 60 inch length of tape approximately only 28 inches is available for recording.
Thus, it is a primary object of the present invention to provide a tape cassette for a scratch strain recorder which makes a much more efficient use of the tape while at the same time not compromising on accuracy and smoothness of operation.
Another object of the invention is to provide a subassembly containing the tape which may be removed when the tape is used up and replaced in the field.
It is a further object to provide a novel tape cassette, particularly but not exclusively, for a scratch strain recorder.